GOLPH3 Pathway Regulation of Golgi Structure and Function PROJECT SUMMARY Secretory trafficking through the Golgi to the plasma membrane is responsible for the proper placement of most of the constituents of the human body. Proteins that depend on trafficking from the Golgi to the plasma membrane play diverse roles in construction of the extracellular matrix that makes up most of our mass, but also in intercellular signaling, mediating exchange of materials across membranes, and cellular adhesion. Given the importance of Golgi-to-plasma membrane trafficking to normal physiology, it is likely that this process is highly regulated with inputs that reflect the status of the cell. Surprisingly, our current understanding of trafficking out of the Golgi provides little insight into the regulation of this process, nor insight into how dysregulation of the Golgi contributes to human disease. The long-term goal of this proposal is to understand the regulation of Golgi structure and function in normal physiology and in disease. Our discovery of the PI4P/GOLPH3/MYO18A/F-actin pathway that plays an important role in Golgi-to-plasma membrane trafficking (Dippold et al., Cell 2009) provides new insight into the biology of the Golgi. Importantly, this pathway has revealed novel regulation of the Golgi. For example, Golgi morphology and secretory trafficking were recently demonstrated to be regulated in response to growth factor signaling via regulation of PI4P levels at the Golgi, thereby regulating the GOLPH3 pathway (Blagoveschchenskaya et al., J Cell Biol 2008). We recently demonstrated that DNA damage results in surprising and dramatic regulation of the Golgi via phosphorylation of GOLPH3 (Farber-Katz et al., Cell 2014). These examples suggest that the GOLPH3 pathway may function as a hub for convergent regulation of the Golgi in response to inter- and intracellular signals. Recent evidence also indicates that the GOLPH3 pathway links regulation of the Golgi to important human disease. For example, we demonstrated that GOLPH3 and MYO18A are each required for propagation of the hepatitis C virus (Bish et al., J Biol Chem 2012). More surprisingly, GOLPH3 and MYO18A drive cancers in humans (recently reviewed in Buschman et al., Cancer Research 2015). Together, the data argue that the GOLPH3 pathway is an important node in the regulation of Golgi morphology and secretory function with important biological significance. Here we propose to extend our studies of the PI4P/GOLPH3/MYO18A/F-actin pathway that we discovered to illuminate novel regulation of Golgi structure and function, to identify signaling pathways that effect this regulation, and to provide insight into their dysregulation in human disease.